Avgas 100 This was the standard high octane fuel for aviation piston engines and has a high lead content. There are two major specifications for Avgas 100. The ASTM D 910 and UK DEF STAN 91-90. These two specifications are essentially the same, but differ over antioxidant content, oxidation stability requirements and max lead content.
Avgas 100 is dyed green and is now only produced in a few refineries in the world.
Avgas 100LL This grade is the lower lead version of Avgas 100. Low lead is a relative term. There is still up to 0.56 g/litre of lead in Avgas 100LL. This grade is listed in the same specifications as Avgas 100, namely ASTM D 910 and UK DEF STAN 91-90.
Avgas 100LL is dyed blue and is the main grade of Avgas used worldwide.
Avgas 100VLL This grade is the very low lead version of Avgas 100LL, containing a maximum lead concentration of 0.45 g/litre. It is effectively a variant of Avgas 100LL with a restraint on the max lead content. It could be made available as an interim measure prior to the introduction of an unleaded high octane fuel, should it be necessary to address environmental concerns about leaded fuels. This grade is listed in ASTM D 910 and, other than the lower lead content, is constrained by the same specification requirements as Avgas 100LL. It therefore meets the same aircraft approvals and operating limitation requirements as Avgas 100LL meeting ASTM D910.
Avgas 100VLL is dyed blue.
AVIATION FUELS
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Avgas UL82 This grade is intended to comply with the same aircraft approvals as the original motor gasoline (mogas) Supplementary Type Certificate (STC) approvals, but with better compositional and performance control. It is aimed at the low compression ratio engines which do not need the high octane of Avgas 100 and could be designed to run on unleaded fuel. Avgas UL82 is specified in ASTM D 6227. Unlike other Avgas specifications, ASTM D6227 allows the use of some non-hydrocarbon components used in mogas, such as ethers, but, unlike mogas specifications, alcohols are not permitted.
Avgas UL82 is dyed purple.
Avgas UL87 This is a relatively new grade added to ASTM D6227, driven by the need for some light sport engines to have a higher octane fuel than Avgas UL82.
Avgas UL82 is dyed yellow.
Avgas UL91 Compositionally this grade is somewhat comparable with Avgas 100LL but with a zero lead content, which results in a lower octane rating of 91MON. Avgas UL91 is specified in ASTM D7547. Avgas UL91 differs principally from both Avgas UL87 and UL82 not only in the higher octane rating, but in lower vapour pressure (49kPa max compared with 60kPa max in ASTM D6227) and that oxygenates such as ethers are not permitted. In common with all other current Avgas specifications, ASTM D7547 does not permit the use of alcohols such as ethanol.
Avgas UL91 is dyed orange.
History of Avgas Grades Avgas is gasoline fuel for reciprocating piston engined aircraft. As with all gasolines, avgas is very volatile and is extremely flammable at normal operating temperatures. Procedures and equipment for safe handling of this product must therefore be of the highest order.
Avgas grades are defined primarily by their octane rating. Two ratings are applied to aviation gasolines (the lean mixture rating and the rich mixture rating) which results in a multiple numbering system e.g. Avgas 100/130 (in this case the lean mixture performance rating is 100 and the rich mixture rating is 130).
In the past, there were many different grades of aviation gasoline in general use e.g. 80/87, 91/96, 100/130, 108/135 and 115/145. However, with decreasing demand these were rationalised down to one principle grade, Avgas 100/130. (To avoid confusion and to minimise errors in handling aviation gasoline, it is now common practice to designate the grade by just the lean mixture performance rating; thus Avgas 100/130 becomes Avgas 100).
Some years ago, an additional grade was introduced to allow a common fuel to be used in engines originally designed for grades with lower lead contents as well as in those engines certified for higher lead contents. This grade is called Avgas 100LL, the LL standing for ‘low lead’.
All equipment and facilities handling avgas are colour coded and display prominently the API markings denoting the actual grade carried. Currently, the two major grades in use internationally are Avgas 100LL and Avgas 100. To ease identification the fuels are dyed: Avgas 100LL is coloured blue, while Avgas 100 is coloured green.
In 1999 a new Avgas grade UL82 (UL standing for unleaded) was introduced as a low octane grade suitable for low compression engines. It has a higher vapour pressure than conventional Avgas and can be manufactured from motor gasoline components, but, notably, the specification does not allow alcohols such as ethanol to be used. It is particularly applicable to those aircraft which have STCs to use automotive gasoline.
An extension of this has been the grade Avgas UL87, which was created in response to the higher octane demand of some light sport engines; notably the turbocharged Rotax® engines. UL87 is otherwise similar to UL82, using similar components, but again expressly excluding alcohols.
The relatively high vapour pressure of the ASTM D6227 specification makes UL82 and UL87 somewhat unsuitable for high altitude flight as engine failure from vapour lock can be an issue. In order to meet the demands from the military for an unleaded Avgas for use in high flying, unmanned aerial vehicles (UAVs), a new low vapour pressure UL91 grade was introduced, resulting in the requirement for a new specification, ASTM D7547. At the time of writing, this specification is approved for light sport engines, such as Rotax®, and is in the process of being considered for approval in a wider range of general aviation engines of low to mid-octane demand. However, it is clear that this will not be of high enough octane rating to be used safely in all general aviation engines and work continues in trying to find a true unleaded alternative to the almost ubiquitous Avgas 100LL.
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